rmlock(9): add an RM_DUPOK flag

Allows for duplicate locks to be acquired without witness complaining.
Similar flags exists already for rwlock(9) and sx(9).

Reviewed by: markj
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.

(cherry picked from commit 2816bd8442bc323d50434d0e64cb4b7c10a195e8)